Engines, for example vehicle engines, often include aspirators and/or check valves. Typically, the aspirators are used to generate a vacuum that is lower than engine manifold vacuum by inducing some of the engine air to travel through a venturi. The aspirators may include check valves therein or the system may include separate check valves. When the check valves are separate they are typically included downstream between the source of vacuum and the device using the vacuum.
During most operating conditions of an aspirator or check valve the flow is classified as turbulent. This means that in addition to the bulk motion of the air there are eddies superimposed. These eddies are well known in the field of fluid mechanics. Depending on the operating conditions the number, physical size, and location of these eddies is continuously varying. One result of these eddies being present on a transient basis is that they generate pressure waves in the fluid. These pressure waves are generated over a range of frequencies and magnitudes. When these pressure waves travel through the connecting holes to the devices using this vacuum, different natural frequencies can become excited. These natural frequencies are oscillations of either the air or the surrounding structure. If these natural frequencies are in the audible range and of sufficient magnitude then the turbulence generated noise can become heard, either under the hood, and or in the passenger compartment. Such noise is undesirable and new apparatus are needed to eliminate or reduce the noise resulting from the turbulent air flow.